Papers containing aramid floc and fibrids, with or without special ingredients such as quartz fibers, are used as substrates for laminate and composite electronic printed wiring boards. For this end use the paper needs to be strong, easily penetrated by the impregnation resin and resistant to solvents used in the resin impregnation process.
The copper-clad laminates made from such papers need to be dimensionally stable at elevated temperatures for improved performance in the fabrication process for making printed wiring boards.
Aramid papers are generally prepared by a wet process using a paper making machine. Wet-laid sheets are drained, dried and calendered, before they are impregnated with resin. Copper-clad laminates made from these resin impregnated papers may then be converted into printed wiring boards (PWB) for use in electronic applications.
In the process of resin impregnating, the strength of the sheet becomes a critical parameter. Heat treatment processes to improve particular physical properties for aramid molded materials are known. For example, for certain molecular structure aramid polymers, films and molded products made from these polymers, Japanese Patent Application Sho 47(1972)-97740 taught a heat treatment of tens of minutes to days at 180 to 400.degree. C. in air to converted amide to imide bonds.
In the production of aramid papers, Kirayoglu, et al., U.S. Pat. No. 5,223,094 taught a process by which the strength and porosity of aramid papers could be improved, but even with heat treatments such as that taught by Kirayoglu et. al., the solvent resistance of aramid paper was less than desired.
The present invention provides a simple, rapid method not only to improve the solvent resistance of the aramid sheet, but also to improve the dimensional stability of laminates made from these papers.